Why are you having trouble with?
Here's a site that asks random subnetting questions. I used it a lot to verify I knew my shyt.
subnettingquestions.com - Free Subnetting Questions and Answers Randomly Generated Online
Here's a sample question:
Here's the process I go through in my head when subnetting:
Determine the network mask
/25
Use the network mask to determine the subnet of the host
1. Convert the subnet mask (/25) to binary - 11111111 11111111 11111111 10000000
2. Separate the network portion (1's) from the host (0's)
Network - 11111111 11111111 11111111 1
Host - 0000000
3. Since the first 3 octets are all 1's, you know the subnet starts with 192.168.92
4. Convert the last octet from binary to decimal to determine the subnet
10000000 in decimal is 128, so the subnet for this address 192.168.92.128 (If you don't know how to go from binary to decimal, look it up).
192.168.92.128 is the subnet for the IP address
5. To determine the number of valid hosts use the formula 2^n -2 where "n" is the number of host bits
2^7 = 128
128 - 2 = 126
There are a max of 126 available hosts in the 192.168.92.128 subnet
6. To determine the first available host, add 1 to the last octet in the subnet
subnet = 192.168.92.128
add 1 to last octect = 192.168.92.129
192.168.92.129 is the 1st valid host on the subnet
7. To determine the last valid host add the number of valid hosts to the last octect in the subnet
number of valid hosts = 126
Last octect in subnet 192.168.92.128 = 128
126+128 = 254
The last valid host in the subnet is 254 or 192.168.92.254
8. To determine the broadcast address, add 1 to the last valid host in the subnet
Last valid host is 192.168.92.254
Add 1 to last valid host = 192.168.92.255
The broadcast address is 192.168.92.255
Summary:
Host: 192.168.92.137/25
Subnet: 192.168.92.128
First valid host: 192.168.92.129
Last valid host: 192.168.92.254
Broadcast address 192.168.92.255
Available hosts: 126
It's a bit confusing at first, but if you practice for about an hour or so, you should get the idea. For the exam you should be able to subnet in your head, so practice it before you take the exam.